In my '87 560SEL the rear climate control vent (above the ashtray, below the back of the arm rest) blows cold air no matter what settig the climate control is on. This isn't a huge problem, since I usually drive in the front seat, but it is a bit annoying. All other vents (dash, under dash, floor) seem to work fine, just this one with the problem. Any ideas on where I could start on this? Thanks